Recently, with the increase in public awareness of environmental issues such as global warming and others, expectations for solar power generation have been much raised in view of the cleanness thereof and freeness from environmental pollution. A solar cell constitutes the main part of a solar power generation system which directly converts the energy of sunlight into electricity. Regarding the configuration of the solar cell, multiple solar cell elements (simply referred to as cells) are connected in series or in parallel and are packaged for protecting the cells to give individual units. The unit installed in the package is referred to as a solar cell module, and, in general, this is so designed that the face thereof to be exposed to sunlight is covered with a transparent substrate as a top protective material (glass/translucent solar cell sheet; front sheet) and sealed up with an encapsulant material (encapsulant resin layer) of a thermoplastic plastic (for example, ethylene-vinyl acetate copolymer), and the back thereof is protected with a back sealing sheet (back sheet) as a back protective material.
Those solar cell modules are mainly used outdoors, and are therefore required to satisfy various properties in point of the configuration, the material constitution and others thereof. Mainly the above-mentioned encapsulant material is required to have softness and impact resistance for protecting solar cell elements, heat resistance for protecting solar cell modules in heat generation, transparency (whole light transmittance, etc.) for efficient arrival of sunlight to solar cell elements, durability, dimensional stability, flame retardation, water vapor barrier performance, etc.
At present, as an encapsulant material for solar cell elements in solar cell modules, an ethylene-vinyl acetate copolymer (hereinafter this may be abbreviated as EVA) is widely used as the constituent material thereof, from the viewpoint of softness, transparency and the like (for example, see Patent Reference 1). Mainly for imparting heat resistance to EVA, crosslinking is attained with an organic peroxide serving as a crosslinking agent. For this, there is employed a method of previously preparing an EVA sheet mixed with a crosslinking agent (organic peroxide) and a crosslinking promoter and sealing up solar cell elements by the use of the resulting sheet. In the process of preparing the sheet, molding at a low temperature at which the organic peroxide used is not decomposed (in general, 80 to 100° C. or so) is needed, and therefore, it is difficult to elevate the speed of extrusion molding; and the sealing process for solar cell elements requires two steps of a step of degassing and temporarily bonding in a laminator taking from a few minutes to a dozen more minutes and another step of finally bonding (crosslinking) in an oven at a high temperature at which the organic peroxide is decomposed (in general, 130 to 150° C. or so) taking a few minutes to 60 minutes or so. Consequently, the production of solar cell modules requires a number of processes and much time, therefore providing a problem of increasing the production cost.
Another problem with the encapsulant material for solar cell elements using the EVA sheet is that the solar cell circuit may be corroded by acetic acid to be generated by EVA through hydrolysis in long-term use; and in addition, owing to the crosslinking agent, the crosslinking promoter and even the generated acetic acid, there may occur another problem of delamination at the interface to the solar cell element or at the interface to the front sheet, or even at the interface to the back sheet.
Against these problems, for example, Patent Reference 2 discloses a solar cell encapsulant material of a polymer blend or a polymer alloy that comprises at least one polyolefin-based copolymer and at least one crystalline polyolefin, as a solar cell encapsulant material not using an EVA sheet and not requiring a crosslinking step.
Patent Reference 3 discloses a solar cell encapsulant material that contains a propylen-based polymer having a melting point of 100° C. or higher and an ethylene-α-olefin copolymer, etc.